Age of Empires Series Wiki

Intensive battle training gives your Heroes more hitpoints and attack.
—In-game description

Tlaxcala Initiation (Aztec Initiation before update 61213) is a technology in Age of Empires III: Definitive Edition available to the Mexicans at the Native Embassy after aging up with Tlaxcala and sending the 'Tlaxcala Alliance' Home City Card. In vanilla Age of Empires III, it can be researched at a Trading Post built on an Aztec settlement. Once researched, it increases the hit points and attack of hero units by 100%.

This technology was, to some degree, replaced by Sikh Gurus in The Asian Dynasties, before it was fully reintroduced in the Mexico DLC.